Magnesium-base alloy



United States Patent 3,131,095 MAGNESIUM-BASE ALLOY Gordon F. Hershey, George S. Foerster, and Sidney L. Couling, Midland, Mich, assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Filed Apr. 10, 1961, Ser. N 101,665 2 Claims. (1. 148-32) Zinc 1.5 to 3.25%. Rare earth metal 0.7 to 1.5%. Zirconium 0.05 to 0.8%. Magnesium Substantially the balance.

Generally, at low zinc levels, e.g., below about 1.5% of zinc, the Mg-Zn-R.E.-Zr alloys exhibit low strength properties. As the zinc level of the alloy is increased above about 3.5% and higher, the alloy exhibits poorer weldability, a tendency to discolor on being extruded, and an increased tendency towards hot shorting on being extruded at economically desirable extrusion speeds, e.g., 8 or more feet per minute.

At low rare earth metal concentrations, below about 0.7% of rare earth metal, these alloys exhibit generally lower strength properties, particularly in the transverse direction of extruding the metal. At rare earth. metal levels above about 1.5% these alloys alsov exhibit greater anisotropy in the transverse and longitudinal directions of extruding, are less ductile and more brittle and require the use of greater extrusion pressures in transforming billets of the metal into extruded product.

Generally the strength properties of the alloy are increased upon employing increasingly greater amounts of zirconium above about 0.05%.

In a more desired range of compositions the alloy con tains 2 to 3.25% of zinc, 0.7 to 1% of rare earth metal,

0.1 to 0.6% of zirconium, the balance being substantially magnesium. The optimum alloy composition is about 3% of zinc, about 0.9% of rare earth metal, from 0.2 to 0.4% of zirconium, and the balance substantially magnesium.

Rare earth metals suitable for use in preparing the present alloy are Ce, La, Pr, Nd, or misch metal (a mixture of rare earth metals). Misch metal containing from 35 to 80 percent of cerium, the balance being rare earth metal and up to 5 percent of non-rare earth metal, is the preferred rare earth metal ingredient of the alloy. Any of the foregoing rare earth metals may be used alone or in any combination in compounding the alloy.

The alloy may be made in the desired proportions according to the invention by melting together the alloying ingredients in proper proportions or by using hardeners of magnesium alloys containing the alloy constituents. Protection from oxidation during alloying is effected by the use of a magnesium chloride-free saline flux as in conventional alloying of magnesium. The molten alloy may be flux refined, if desired, by stirring the alloy With additional flux. The so-refined metal is allowed to settle and then is separated from the flux as by decanting into a suitable casting mold, e.g., a round mold for extrusion stock.

In extruding the cast metal, it is desirable first to scalp the cast metal so as to present a smooth clean surface to the extrusion die. The clean extrusion stock is heated to a suitable temperature, e.g., about 700 to 900 F. The heated metal is then extruded in a conventional metal extrusion press.

The following examples are given to illustrate the present invention but are not to be construed as limiting the invention thereto.

EXAMPLE Various alloy compositions Were cast into billets 3 inches in diameter, scalped to 2% inches diameter, cut into 4 inch lengths, heated to between 750 and 900 F. and extruded at a speed of 17 to 18 feet per minute from the 3 inch container of a 500 ton press into a 0.090 by 2.5 inch strip. Some of the strip was machined into test bars in both the longitudinal and transverse directions of extruding, and tensile yield strength and compression yield Table I Composition, Weight Tensile Yield Strength 2 Compression Percent of Metal Extruded at- Yield Test N0. Condition Direction Strength 2 of metal of Test of metal Zn Zr MM 750 F 800 F. 850 F. Extruded at 24, 000 25, 000 25,000 1 ,00 1 92 92 i A {L 28,000 26,000 25, 000

E 27, 000 28, 000 29, 000 19, 000 F {T 2 3. l5 0. lo 0. 86 L s, A L 3 1.56 0. 66 1.10 s, A

Comparison 1 0.5 0. 4 0.5 F Comparison 2 3. 20 0. l0 0. 61 L s, A

F Comparison 3. 3.07 0. 74 L s, A

1 Balance commercial magnesium.

2 All yield strengths in p.s.1. M M =misch metal. F=as extruded.

strength tests were carried out at room temperature. Additional pieces of strip were placed in the jaws of apparatus adapted to straighten the strip by stretching it. The strip was stretched at room temperature in a longitudinal direction until the center portion, midway the jaws of the apparatus, had elongated (nominally) about 1.5%. The stretched strips were then aged about 16 hours at 400 F. Test bars were machined from the aged strip in both the longitudinal and transverse directions of extruding and similarly tested at room temperature for tensile yield strength and compression yield strength. The test results are given in Table I.

By way of comparison, several alloys having compositions outside the scope of the invention were similarly extruded and tested. The results of the comparison tests are shown in Table I.

As a further comparison a commercial alloy widely used in extruded form and having the nominal composition 3% of aluminum, 1% of zinc, at least 0.15% of manganese, and the balance magnesium, was cast as a billet, scalped, heated and pre-extruded in the form of a 3 inch diameter billet, further heated to an extrusion temperature in the range of 650 to 850 F. and pushed from a 500 ton extrusion press having the container heated to about the same temperature as the metal. The metal was extruded at about 17 to 18 feet per minute as a strip 0.090 by 2.5 inches in cross-section. Test bars were cut from the strip in both the longitudinal and transverse directions of extruding and tested for tensile yield strength at room temperature. The test data are listed in Table II. The data show that the transverse properties of this commercial alloy are adversely afiected on increasing the extrusion temperature in constrast to the alloy of the invention.

L=longitudinal direction of extruding.

T=transverse direction of extruding.

To demonstrate the weldability of the alloy, compositions according to the present invention were cast into billets and rolled, while at an elevated temperature, into inch thick sheet. Strips 8 to 12 inches long and about inch wide were cut from the sheet and used to lay a weld bead (electric are under helium atmosphere) about the circumference of a 3 inch diameter circle centrally disposed on an 8 inch by 8 inch work piece cut from the same sheet. On allowing the weld bead to cool, the length of cracks developing along the bead were measured. The length of the crack along a weld was divided by the Table III Composition, Weight Percent 1 Percent of Test N0. Cracking Zn Zr MM 2. 9 0. 8 1. 19 4. 8 3. 2 0. 7 0. 89 7. 4 3.0 0. 8 0.88 9. 1 Comparison 4 3. 0 0.8 43 Comparison 5.. 3.0 0. 7 0. 32 63 Comparison 6.- 3.0 0. 7 0. 14

1 Balance commercial magnesium. MM =misch metal.

Among the advantages of the present alloy are: (1) the high level of strength properties in the transverse direction of extruding whereby the alloy exhibits generally isotropic properties, especially the tensile yield strength, in both the longitudinal and transverse direction of extruding, and (2) the alloy may be extruded at unusually high temperatures Without substantial loss in strength properties, whereby lower extrusion pressures may be employed.

We claim: 1. A magnesium-base alloy in extruded form which consists essentially of, by weight:

from 1.5 to 3.25 percent of zinc, from 0.7 to 1.5 percent of rare earth metal, from 0.05 to 0.8 percent of zirconium, the balance magnesium; said alloy having been pushed through an extrusion die while the alloy was at a temperature in the range of about 700 to 900 F.; and said alloy having a tensile yield strength of at least 23,000 pounds per square inch in the transverse direction of extruding. 2. The alloy as in claim 1 in which the rare earth metal is misch metal.

References Cited in the file of this patent UNITED STATES PATENTS 2,979,398 Foerster Apr. 11, 1961 FOREIGN PATENTS 165,802 Australia Oct. 27, 1955 205,975 Australia Jan. 25, 1957 OTHER REFERENCES Busk: Magnesium Alloys, taken from Precipitation From Solid Solution, A.S.M., Cleveland, Ohio, 1959, pages 409412.

Mellor et al.: The Creep Strength at 200 C. of Some Mg Alloys Containing Ce, The Journal of the Institute of Metals, vol. LmV, 1948-1949, page 692. 

1. A MAGNESIUM-BASE ALLOY IN EXTRUDED FORM WHICH CONSISTS ESSENTIALLY OF, BY WEIGHT: FROM 1.5 TO 3.25 PERCENT OF ZINC, FROM 0.7 TO 1.5 PERCENT OF RARE EARTH METAL, FROM 0.05 TO 0.8 PERCENT OF ZIRCONIUM, THE BALANCE MAGNESIUM; SAID ALLOY HAVING BEEN PUSHED THROUGH AN EXTRUSION DIE WHILE THE ALLOY WAS AT A TEMPERATURE IN THE RANGE OF ABOUT 700 TO 900*F.; AND SAID ALLOY HAVING A TENSILE YIELD STRENGTH OF AT LEAST 23000 POUNDS PER SQUARE INCH IN THE TRANSVERSE DIRECTION OF EXTRUDING. 